Hydraulic jack synchronizing system



g- 1965 w. H. SCHlLLING ,26

HYDRAULIC JACK SYNCHRONIZING SYSTEM Filed April 50, 1964 2 Sheets-Sheet1 Aug. 9, 1966 w. H. SCHILLING HYDRAULIC JACK SYNCHRONIZING SYSTEMSheets-Sheet 2 United States Patent 3,265,357 HYDRAULIC JACKSYNCHRONIZENG SYSTEM William H. Schilling, La Hahra, Califi, assignor toHydraulic Engineering orporation, El Monte, Califi, a corporation ofCalifornia Filed Apr. 30, 1964, Ser. No. 363,824 9 Claims. (Ql. 254-8)This invention relates to a hydraulic jack synchronizing system which isan improvement on, and simplification of, the valving arrangement ofBorn Patent No. 2,984,072. A load-supporting jack assembly iscontemplated which has operatively associated therewith automaticsynchronizing and hydraulic fluid equalizing valves to synchronize themovements of two or more hydraulic jacks acting in parallel to advancethe load along a predetermined path without deviation therefrom anddespite unbalanced loading of the jack units or any shifting of the loadon the jacks.

Hydraulic jacks and other load-moving assemblies have many applicationsrequiring the use of two or more jack units acting in conjunction witheach other to support a load and wherein it is mandatory that themovement of all cylinders be positively and precisely synchronized bothduring extension and retraction of the jacks. By way of illustration ofsuch needs, there may be mentioned the elevation and lowering of theroof of a missile-tracking station wherein in one position the roofcovers a block house and its contained tracking instrument, and inanother position the roof is moved horizontally onto a jacksupportedtrack after which the track is lowered to position the roof clear of thetracking instrument. In such installations the roof corners shouldremain level to better than plus or minus A" on a 30 foot diagonalduring roof-down. or roof-up operations. Other uses are the elevation ofprecast concrete floors from their casting position at ground level totheir assembled position atop vertical supporting columns and theraising of the ends of a bridge which are on opposite shores of a river,it being desirable to synchronize the operation of jacks at each end tokeep the span level as it is raised.

Various attempts have been made heretofore to provide a hydraulic jacksystem utilizing a plurality of hydraulic cylinders arranged in paralleland capable of extension or retraction under precisely controlledconditions. The Born patent above referred to discloses a satisfactoryjack assembly and valve mechanism for that purpose. The presentinvention contemplates an extension of the Born disclosure particularlyin the field of providing a valving arrangement specifically designedfor keeping a load-raising platform or the like level.

One object of the present invention is to provide a hydraulic jacksynchronizing system wherein a load-supporting element is raised andlowered by at least two hydraulic jacks operatively connected therewithat spaced-apart points to move the load-supporting element away from andtoward a support, fluid pressure supply means being connected to bothjacks for supplying fluid pressure thereto, and means being provided formaintaining a predetermined angle between the load-supporting elementand the support comprising a synchronizing and pressure equalizing valvefor each jack and an angle deviationsensing means operable to adjustthese valves for stopping Flow of fluid pressure from the supply meansto one jack when ahead of the other, the arrangement being quitesensitive to minute deviations to thereby minimize such deviations.

Another object is to provide a synchronizing and pressure equalizingvalve for each jack comprising a valve body having therein opposite andaligned valve seats and first and second cavities outside of theseseats, a stem extending through the seats and cavities and having avalve 3,265,357 Patented August 9, 1966 head between the seats of lesslength than the distance between them and which is operable to seat onthe valve seats one at a time, there being a pump inlet to bothcavities, a check valve between the pump inlet and each of the cavitieswith one check valve opening away from the first cavity and the otheropening toward the second cavity and the angle deviation-sensing meansbeing operable to move the valve head in the direction of the secondcavity when its corresponding jack is ahead of the other and in thedirection of the first cavity when its corresponding jack is behind theother, the valve body having a central cavity between the valve seatscommunicating with the jack.

Still another object is to provide a synchronizing valve mechanism in asystem of the character described having a novel linkage arrangement andvalve actuating mechanism for sensing deviations of any one or morejacks of a plurality of jacks, and for automatically readjusting theflow of hydraulic fluid to or from the jacks for maintaining a desiredattitude of the load being lifted.

A further object is to provide the actuating mechanism with means forpermitting override with respect to any one or more of the synchronizingvalves being actuated.

Still a further object is to provide the linkage mechanism designed soas to amplify the deviations of the load with respect to the jacksthereby providing a highly sensitive deviation-sensing means and ahighly sensitive synchronizing valve system responding thereto.

With these and other objects in view, my invention consists in theconstruction, arrangement and combination of the various parts of myhydraulic jack synchronizing system, whereby the objects abovecontemplated are attained, as hereinafter more fully set forth, pointedout in my claims and illustrated in detail on the accompanying drawing,wherein:

FIG. 1 is a perspective View of a hydraulic jack synchronizing systemembodying my invention and showing the same applied to lifting means inthe form of tracks for the roof of a missile-tracking station;

FIG. 2 is a diagrammatic view according to the line 2--2 of FIG. 1showing an exaggerated tilted position to illustrate the operation ofangle deviation-sensing valve actuating mechanism;

FIG. 3 is a sectional View corresponding to a portion of FIG. 2 showingdetails of the valve actuating mechanism;

FIG. 4 is an enlarged vertical sectional view on the line 4-4 of FIG. 3showing the valve in one operated position;

FIG. 5 is a sectional view similar to FIG. 4 showing another portion ofthe valve structure in another oper ated position;

FIG. 6 is a sectional view on the line 6-6 of FIGS. 4 and 5, FIG. 4being also a section on the line 44 of FIG. 6 and FIG. 5 being a sectionon the line 55 thereof;

FIG. 7 is a complete hydraulic diagram of the hydraulic jacksynchronizing system and fluid pressure supply and control meanstherefor, and

FIG. 8 is a diagrammatic view combining FIGS. 4 and 5 to show thecomplete hydraulic circuitry through the synchronizing and flowequalizing valve.

On the accompanying drawings I have used the reference numeral 10 toindicate a hydraulic cylinder, four of which are shown by Way ofexample, 12 the pistons therein and 14 the piston rods extending fromthe pistons. A load-supporting element in the form of rails 16 isprovided supported on the upper ends of the cylinders 10 whereas thelower ends of the piston rods 14 are provided with anchor flanges 18adapted to be secured to a floor or other support such as shown at 20 inFIG. 2. Suitable ball and socket joints or their equivalent are providedbetween the upper ends of the cylinders 10 and the rails 16, and betweenthe lower ends of the piston rods 14 and the flanges 18 to avoid anybinding which would interfere with proper operation of the jack systern.The tracks 16 may be provided for the roof of a missile-trackingstation, or the like or in place of the rails 16 a lifting platform orany suitable load-supporting element may be provided.

Referring to FIGS. 2 and 3 synchronizing and flow equalizing valves areshown generally at V mounted as by flanged sleeves 22 screwed thereintoafter passing through a supporting wall 24 of a housing 26 having anextension portion 28 mounted on brackets 30. The brackets 30 are mountedon the rails 16 in any suitable manner and serve to rigidly support thehousing 26-28 relative thereto.

The valves V are provided with valve actuating pins 32 which in turn areactuated by a tilt shaft 34 journaled in a spherical bearing 36 andhaving a primary actuating plate 38 and a secondary actuating plate 40.The plates 38 and 40 are bolted together at 42 and overtravel springs 44are provided on the bolts for a purpose which will hereinafter appear.The secondary actuating plate 40 has a set screw 46 for each valve pin32 and a lock nut 48 to retain the adjustment of the set screw once itis made.

In FIGS. 2 and 3 I illustrate two valves V for two jacks 10-1244 whereasin FIG. 1 four jacks are shown. Two additional valves V are provided forthe other two jacks of FIG. 1 in the same manner as illustrated in FIGS.12 and 3. Some installations may required only two jacks whereas onothers there may be three jacks in which case there would be threevalves, and on still others there might be more than four jacks. In anycase there would be a valve V for each jack.

With respect to the construction of the synchronizing and flowequalizing valve V, reference is made to FIGS. 4, 5, 6 and 8. A valvebody tl 52 is illustrated, the parts 50 and 52 being bolted together forconvenience of assembly. Within the valve body 5052 is a first valveseat 54 and a second valve seat 56 opposed to each other, and outward ofwhich are first and second cavities 58 and 60. A pump inlet 62 leads toboth cavities 58 and 60 as shown in FIG. 8. As shown in FIGS. 4 and 6this is accomplished by a passage 62 to a bore 62* shown in FIGS. 5 and6. There are check valves 64 and 66 between the pump inlet and thecavities, the check valve 64 opening away from the first cavity 58 andthe check valve 66 opening toward the second cavity 64). Between thevalve seats 54 and 56 a central cavity 68 is provided, and communicatingwith this cavity is a valve outlet 70 to the jack.

Within the valve body 50-52 is a valve stem 72 having an enlargedspool-like valve head 74 thereon of a few thousandths of an inch lesslength than the distance between the seats 54 and 56, and having O-ringseals 73. As shown in FIGS. 3 and 8 the head 74 may be centered betweenthe seats 54 and 56, seated on the seat 56 as shown in FIG. 4 when thehead 74 is depressed by pressure against the upper end of the valve pin32 or seated against the seat 54 (FIG. 5) when the position of the tiltshaft 34 is such that pressure is released from the valve pin in whichcase a spring 76 in the body portion 52 effects the valve seatingoperation as between the valve head 74 and the valve seat 54. Obviouslythe total travel of the head is only a few thousandths of an inch, thisdistance being shown exaggerated in FIGS. 4, 5 and 8. On the head 74 inFIG. 8 are a pair of arrows S and L. When the valve head is centered asin FIG. 8, the total length of the valve V and its pin 32 is asindicated at D (also both valves in FIG. 3). S indicates shortening ofthe overall length of the valve and pin as to the dimension D for theright-hand valve shown in FIG. 2 and in FIG. 4 whereas L indicateslengthening of this dimension to the length D" shown for the lefthandvalve in FIG. 2 and in FIG. 5.

Referring to FIG. 3 the linkage for actuating the tilt shaft 34comprises a slide bearing 78 to which is pivoted the lower end of a link80 for each cylinder 10, the upper end of the link being pivoted toanother link 82. The lower end of the link 82 is pivoted to a stationarybracket 84. A control rod 86 has its inner end pivoted to the pivotalconnection between the links 80 and 82 and its outer end pivoted to aclamp 88 secured to the cylinder 19. A link, bracket and control rodarrangement such as just described is provided for each jack 184244 inthe system. A linkage arrangement of this kind magnifies the deviationof angle as between the plane of the rails 16 and the jacks to make themovements of the actuating plates 38 and 4t} and thereby'the valves Vextremely sensitive to such deviation.

Referring to FIG. 7 wherein the complete hydraulic system is shown, thejacks and the synchronizing and equalizing valves have already beendescribed. The pump inlets 62 are connected to a manifold 90 which issupplied with fluid under pressure from a hydraulic pump PF which may bedriven as *by an electric motor EM and has the usual pressure reliefvalve 92 to determine the operating pressure of the pump PF and returnhydraulic fluid to the tank whenever a control valve 94 is in positionshutting off the pump from the hydraulic system. The control valve 94has three positions as indicated UP, STOP and DOWN depending on theposition of a control lever 96 for the valve. In the right-hand positionshown fluid pressure is supplied to the system. In a centered positionthe hydraulic system is locked against return flow of fluid whereas inthe left-hand position hydraulic fluid may (by responding to the load onthe jacks) flow from the system back to the tank T.

Practical operation During the installation of .my hydraulic jacksynchronizing system, the valves V require adjustment to maintain thedesired angle of the load-supporting element 16 relative to the support20. In most instances, the support 20 would be level and it would bedesired that the load-supporting element 16 also be maintained levelduring raising and lowering operations. First the load-supportingelement 16 is leveled and then the set screws 46 adjusted until thevalve heads 74 are midway between the seats 54 and 56 (dimenison D). Thelock nuts 48 are then tightened to maintain the adjustment.

After the complete hydraulic system is installed according to FIG. 7,the control lever 96 would normally be in the STOP position so that theoutput from the pump PF would flow back to the tank T through the reliefvalve 92. The hydraulic system above the control valve 94 at this timeis completely locked against flow either inward or outward with respectto the system.

When it is desirable to raise the load-supporting element 16, thecontrol lever 96 is shifted to the UP position shown in FIG. 7 whereuponthe fluid pressure available from the pump PF as limited by the reliefvalve 92 enters the system by flowing into the manifold 90 and fromthence to each of the valves V. As long as the valves remain centered(dimension D, FIGS. 3 and 8) fluid pressure flows to all four cylinders10 of the jacks and the load-supporting element remains level.

As soon as there is even slight movement of one of the jacks ahead ofthe others the valves will be adjusted to reduce or stop flow ofhydraulic fluid to that jack and proportionally increase flow to theothers as required to 'bring the load-supporting element back intobalance at a level attitude. In FIG. 2 this situation is shownexaggerated, the right-hand jack being ahead of the lefthand jack andaccordingly tilting the actuating plate 38- 40 clockwise relative to theload-supporting element 16, thereby depressing the valve pin 32 of theright-hand valve and releasing the pin 32 of the left-hand valve so thatthe right-hand valve head '74 is seated on the seat 56 as shown in FIG.4 and the left-hand valve head is in the left-hand valve, of course,because the set screw 46 can freely leave the pin 32.

The tilted position of the load-supporting element 16 in FIG. 2 is showngreatly exaggerated whereas even .001 movement of the valve head 74upwardly or downwardly will change the fiow ratio to the differentcylinders to bring the load-supporting element 16 back to level positionbefore the valve head can seat on either of the seats 54 or 56. Theremay be a sudden shift of load, however, that would permit such seatingwhich, of course, calls for more drastic equalizing action accomplishedby cutting off all flow to that jack cylinder which is ahead of theothers. Thus the lead jack is then held back while the others catch up.

On the other hand, should the system be in the lowering cycle thepressure from the pump has been eliminated so that oil can now flow backfrom the jack through the check valve 64 as shown by the dotted arrow 98in FIG. 8. Should a certain jack be behind the others in raising orahead in the lowering cycle, the valve assembly will become longer (L)causing the valve head 74 to seat on the seat 54. This permits oil toflow from the pump through the check valve 66 to the jack on the raisingcycle but prevents back flow on the lowering cycle.

From the foregoing specification it will be obvious that I have provideda hydraulic jack synchronizing system in which two or more hydraulicjacks are synchronized by a simple valve having a single valve headmovable between opposite valve seats and provided with check valvesarranged in such a way that flow between the jack and the pump isstopped or permitted depending on the automatic adjustment of thesynchronizing valve as it responds to angle-deviation-sensing means inthe form of linkage that actuates the valve. The several valves meterthe oil to the jacks in direct proportion to the requirements forkeeping the load level or at any desired angle for which theangle-deviation-sensing and valve-actuating mechanism is set. Aspool-type of valve is used having metal-to-metal seats and thearrangement has a relatively long operation life as the valve head isseated by relatively light spring pressure afforded by the springs 44and 76 and the provisions described for overtravel. In most operationsthe spool 74 of the valve floats between the valve seats without evercontacting either of them which, of course, further contributes to along operating life for the valve. The check valves 64 and 66 arearranged parallel and back-to-front to prevent back flow from the jackduring an extension phase and permit metered return of oil during aretraction cycle. When dimension D is shortened (S) as to the dimensionD, no oil can pass from the pump to the jack.

My synchronizing system is outstanding because of simplicity and ease ofinstallation. In summary, it synchronizes the action of two or morehydraulic cylinders so that all cylinders maintain the same rate ofspeed regardless of the distribution of load on the cylinders. Itsoperation can best be described by using as an example the lifting of aplatform by four cylinders, one at each corner of the platform. Whenpressure is applied to the cylinders, the platform moves upward, eachcorner moving in unison with each other corner so that the platformremains perfectly level during the entire lifting operation. When thepressure is relieved each cylinder keeps in perfect step with each othercylinder and the entire platform moves downwardly maintaining at alltimes a perfectly level attitude. This result is achieved by measuringthe angle between the cylinder and the platform which should be if theplatform is to remain level. Whenever the angle increases above ordecreases below 90 a simple synchronizing valve increases or decreasesthe flow of hydraulic fluid to provide the necessary correction. Thesystem is entirely independent of the loading of the platform and of thediameter of the cylinders. The system will work perfectly even ifdifferent pumps are used on each cylinder which can be an advantage ifthe platform is a bridge in which two cylinders are separated from twoother cylinders by a river.

Accuracy within a fraction of an inch of levelne-ss for each 10 feet oflength can easily be attained. By holding the parts to closer tolerancesand applying superfinish techniques, a supersensitive synchronizingvalve for accuracy of a few thousandths of an inch per 10 feet of lengthcan be achieved.

Some changes may be made in the construction and arrangement of theparts of my hydraulic jack synchronizing system without departing fromthe real spirit .and purpose of my invention, and it is my intention tocover by my claims any modified forms of structure or use of mechanicalequivalents which may reasonably be included within their scope.

I claim as my invent-ion:

1. In a hydraulic jack synchronizing system, a loads-upporting element,at least two hydraulic jacks operatively connected therewith a-t spacedapart points to move said load-supporting element away from and toward asupport, fluid pressure supply means connected to both jacks forsupplying fluid pressure thereto, means for maintaining a predeterminedangle between said load-supporting element and the support comprising asynchonizing valve for each jack, angle deviation sensing means operableto adjust said valves for reducing the flow of fluid pressure from saidsupply means to one of said jacks when ahead of the other, each of saidvalves comprising a valve body, opposed valve seats and first and secondcavities therein outward of said valve seats, a central cavity betweensaid seats, a stem extending through said valve seats and cavities andhaving a valve head between the seats of less length than the distancebetween them, a pump inlet to both of said first and second cavities, acheck valve bet-ween said pump inlet and each of said first and secondcavities, one check valve opening away from said first cavity and theother opening toward said second cavity, said angle deviation-sensingmeans being operable to move said valve head in the direction of saidsecond cavity when its corresponding jack is ahead of the other and inthe direction of said first cavity when its corresponding jack is behindthe other, and an outlet to the jack from said central cavity.

2. A system according to claim 1 wherein said angle deviation-sensingmeans comprises a shaft mounted for tilting movement, an operativeconnection from said shaft to each of said synchronizing valves tosimultaneously actuate them in opposite directions, a slide bearing onsaid shaft and a link connection from said slide bearing to each of saidhydraulic jacks.

3. A system according to claim 2 wherein each of said link connectionscomprises a first link having one end pivota-lly mounted on saidload-supporting element and extend-ing substantially parallel to saidshaft, a second link pivoted to the free end of said first link and tosaid slide bearing, said second link extending in an angular directionfrom said first link to said slide bearing, and a third link extendingsubstantially parallel to said load supporting element and connectedwith a hydraulic jack to sense deviation of the angle between it andsaid loadsupporting element.

'4. A system according to claim 2 wherein said operative connectionincludes two elements and spring means to normally effect movement ofone simultaneously with the other, said spring means permitting overrunmovement of the element connected with said shaft when the second oneeffects seating of said synchronizing valve.

5. In a hydraulic jack synchonizing system, a loadsupporting element, atleast two hydraulic jacks operatively connected therewith at spacedapart points to move said load-supporting element away from and toward asupport, fluid pressure supply means connected to both jacks forsupplying fluid pressure thereto, means for maintaining a predeterminedangle between said load-supporting element and the support comprising asynchonizing valve for each jack, angle deviation-sensing means operableto adjust said valves for reducing the flow of fluid pressure from saidsupply means to one of said jacks when ahead of the other, each of saidvalves having opposed Valve seats, a valve head normally positionedbetween said seats and out of contact therewith, a pump inlet to :bothof said seats, an outlet to the jack from between said sea-ts, a checkvalve between said pump inlet and one of said seats, a check valvebetween said pump and the other of said seats, said check valvesopening, one toward its valve seat and the other away from its valveseat, means biasing said valve head toward the first of said last twomentioned seats and said deviation-sensing means being operable to movesaid valve head again-st such bias and toward the second of said valveseats when the jack corresponding to that particular valve moves aheadof the other jacks.

6. A system according to claim wherein said angle deviation-sensingmeans comprises a tilt shaft, an operative connection from said tiltshaft to each of said synchronizing valves to simultaneously actuatethem in opposite directions, a slide bearing on said shaft and a linkconnect-ion from said slide bearing to each of said hydraulic jacks.

7. A system according to claim 6 wherein said link connections comprisefirst links having one end pivotally mounted on said load-supportingelement and extending substantially parallel to said shaft, second linkspivoted to the free ends of said first links and to said Slide bearing,said second links extending in angular directions from said first linksto said slide bearing, and third links extending substantially parallelto said load-supporting element and connected with said hydraulic jacksto sense deviations of angle between them and said load-supportingelement.

8. A system according to claim 5 wherein said angle deviation-sensingmeans comprises a tilt shaft, an operative connection from said shaft toeach of said synchronizing valves to simultaneously actuate them inopposite di rections, a slide bearing on said tilt shaft, and linkconnections from said slide bearing to said hydraulic jack, saidoperative connection comprising a primary arm and a secondary arm, saidprimary arm being carried by said tilt shaft, said secondary arm beingnormally in engagement with said primary arm and cooperable with saidsynchronizing valves to actuate them, and spring means biasing saidsecondary arm into engagement with said primary arm.

9. In a hydraulic synchonizing system, a load-supporting element, atleast two hydraulic jacks operatively connected therewith at spacedapart points to move said load-supporting element away from and toward asupport, fluid pressure supply means and control valve means connectedto both jacks for supplying fluid pressure thereto or permitting returnof fluid pressure therefrom, means for maintaining a predetermined anglebetween said loadsupporting element and said jacks comprising asynchronizing valve for each jack and angle deviation-sensing meansoperable to adjust said valves for reducing the flow of fluid pressurefrom said supply means to one of said jacks when it is ahead of theother and for increasing the flow of fluid pres-sure from said supplymeans to said one of said jacks when it is behind the other, each ofsaid valves comprising a valve body, opposed valve seats and first andsecond cavities therein outward of said valve seats, a central cavitybetween said seats, a stem extending through said valve seats andcavities and having a valve head between the seats of less length thanthe distance between them, a pump inlet to both of said first and secondcavities, a check valve between said pump inlet and each of said firstand second cavities, one check valve opening away from said first cavityand the other opening toward said second cavity, said angledeviationsensing means being operable to move said valve head in thedirection of said second cavity when its corresponding jack is ahead ofthe other and in the direction of said first cavity when itscorresponding jack is behind the other, and an outlet to the jack fromsaid central cavity.

WILLIAM FELDMAN, Primary Examiner. OTHELL M. SIMPSON, Examiner.

1. IN A HYDRAULIC JACK SYNCHRONIZING SYSTEM, A LOADSUPPORTING ELEMENT,AT LEAST TWO HYDRAULIC JACKS OPERATIVELY CONNECTED THEREWITH AT SPACEDAPART POINTS TO MOVE SAID LOAD-SUPPORTING ELEMENT AWAY FROM AND TOWARD ASUPPORT, FLUID PRESSURE SUPPLY MEANS CONNECTED TO BOTH JACKS FORSUPPLYING FLUID PRESSURE THERETO, MEANS FOR MAINTAINING A PREDETERMINEDANGLE BETWEEN SAID LOAD-SUPPORTING ELEMENT AND THE SUPPORT COMPRISING ASYNCHRONIZING VALVE FOR EACH JACK, ANGLE DEVIATION-SENSING MEANSOPERABLE TO ADJUST SAID VALVES FOR REDUCING THE FLOW OF FLUID PRESSUREFROM SAID SUPPLY MENAS TO ONE OF SAID JACKS WHEN AHEAD OF THE OTHER,EACH OF SAID VALVES COMPRISING A VALVE BODY, OPPOSED VALVE SEATS ANDFIRST AND SECOND CAVITIES THEREIN OUTWARD OF SAID VALVE SEATS, A CENTRALCAVITY BETWEEN SAID SEATS, A STEM EXTENDING THROUGH SAID VALVE SEATS ANDCAVITIES AND HAVING A VALVE HEAD BETWEEN THE SEATS OF LESS LENGTH THANTHE DISTANCE BETWEEN THEM, A PUMP INLET TO BOTH OF SAID FIRST AND SECONDCAVITIES, A CHECK VALVE BETWEEN SAID PUMP INLET AND EACH OF SAID FIRSTAND SECOND CAVITIES, ONE CHECK VALVE OPEING AWAY FROM SAID FIRST CAVITYAND THE OTHER OPENING TOWARD SAID SECOND CAVITY, SAID ANGLEDEVIATION-SENSING MEANS BEING OPERABLE TO MOVE SAID VALVE HEAD IN THEDIRECTION OF SAID SECOND CAVITY WHEN ITS CORRESPONDING JACK IS AHEAD OFTHE OTHER AND IN THE DIRECTION OF SAID FIRST CAVITY WHEN ITSCORRESPONDING JACK IS BEHIND THE OTHER, AND AN OUTLET TO THE JACK FROMSAID CENTRAL CAVITY.